262 



TfNNELING 



that is, the tail-plates overlap the lining only for, say, the roof 

 from the springing lines up, as in one of the shields for the 

 Clichy tunnel. In unstable materials, the tail-plating ex- 

 tends entirely around the shield and excavation. The length 

 of the (ail-plating is usually sufficient to overlap two rings of 

 the lining, but in one of the Clichy tunnel shields it will be 

 noticed that it extended over three rings of lining. This 

 seemingly considerable space for thin steel plates is made 

 possible by the fact that the extreme rear end of the tail 

 always rests upon the last completed ring of lining. 



In closing these remarks concerning the rear-end con- 

 struction, the accompanying table, prepared by Mr. Raynald 

 Legouez, will be of interest, as a general summary of principal 

 dimensions of most of the important tunnel shields which have 

 been built. The figures in this table have been converted 

 from metric to English measure, and some slight variation 

 from the exact dimensions necessarily exists. The different 

 columns of the table show the diameter, total length, and the 

 length of each of the three principal parts into which tunnel 

 shields are ordinarily divided in construction as previously 

 described : 



